Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: The Messenger of Allah (peace and blessings be upon him) forbade selling fruit before it ripens, and likewise forbade selling wool on the back of an animal (not yet shorn) and milk in the udder (not yet milked).
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11885